1. Field of the Invention
The present invention relates to a method for manufacturing a spiral coil, the spiral coil, and an electro-magnetic acoustic transducer (EMAT) including the same, and more particularly, to a method for manufacturing a spiral coil used in an EMAT, the spiral coil, and an EMAT including the same.
2. Description of the Related Art
Electro-magnetic acoustic transducers (EMATs) widely used in nondestructive inspection fields are classified into two types by ultrasonic wave generation mechanism, i.e., into Lorentz force transducers and magnetostrictive transducers. The Lorentz force transducers use interference between a static magnetic field and an eddy current in a conductive material, and the magnetostrictive transducers use dimension change (magnetostriction) of a ferromagnetic material due to superimposition of a bias static magnetic field and a dynamic magnetic field. The static magnetic field is provided to a material by a permanent magnet or electromagnet, and the eddy current and dynamic magnetic field are provided to a material by a coil operated by an alternating current (AC) pulse in a radio frequency (RF) band, i.e., an RF coil. Ultrasonic waves are generated near a surface of the material due to a skin effect of the eddy current and dynamic magnetic field, and a mode thereof is adjusted by a relative direction of the static magnetic field and eddy current (or dynamic magnetic field). During a receiving process, the RF coil detects a change of magnetic flux aroused in the material by the ultrasonic waves. When a target material is a conductive material or ferromagnetic material, the target material may be used as an element of a transducer so as to generate and detect ultrasonic waves in the target material without having to directly contact the target material to other two elements (RF coil and bias magnet) of the transducer. When the target material is a nonconductive material or nonmagnetic material, ultrasonic waves may be transmitted or received by temporarily or permanently adhering a thin strip having an excellent transducing characteristic to the target material. The thin strip approach may also be used with a target of a conductive or ferromagnetic material for efficient ultrasonic wave transmission and reception.
FIG. 1 schematically illustrates an elongated spiral surface RF coil (having large slenderness) widely used for various EMATs generating or detecting longitudinal waves, transverse waves, or guided waves. When the elongated spiral surface RF coil is used, an EMAT is designed such that an ultrasonic transmitting and receiving area is limited to two parallel leg portions of the elongated spiral surface RF coil. A current optimum method for manufacturing a spiral coil uses a printed circuit technology. The printed circuit technology enables manufacturing of a coil that is very accurate, thin, and flexible, but requires expensive devices. Hirao and Ogi's writings describe in detail about manual manufacturing processes from an enamel copper wire to a spiral surface coil. Such manual manufacturing processes for manufacturing a spiral surface coil are particularly suitable for manufacturing a small coil. However, as a size of spiral surface coil increases, manufacturing costs are remarkably increase or accuracy is deteriorated. Thus, magnetostrictive guided wave transducers used for long-range ultrasonic test of a large structure use a solenoid that has lower guided wave transmitting and receiving efficiency than a spiral surface coil. The solenoid is not a surface coil. Accordingly, the solenoid includes a connector electrically connecting adjacent loops for easy installation to a surface of the large structure, which makes it difficult to variously apply coils having an optimized wound number.